1. Field of the Invention
The invention relates to a process for producing large size formed articles consisting of a modified polyamide by means of anionic polymerization of an omega-lactam, said articles being obtained according to the casting or rotational moulding technologies (see for example NYLON PLASTICS, edited by M. I. KOHAN; John Wiley and Sons Publishers; 1973).
2. Discussion of the Prior Art
The anionic polymerization of an omega-lactam in a mold (according to the casting or rotational moulding technology) is a technique which permits to obtain articles of great dimensions and variable thickness, which otherwise would be difficult and not profitable to obtain by means of the conventional techniques based on the extrusion or injection of the polyamide granules. In fact, it is possible to take advantage of the possibility of filing the mould with a low-viscosity liquid mixture (molten omega-lactam+catalyst+activator) and therefore it is possible to operate with lower injection pressures, with simplier moulds and with a shorter filling time, even if the moulds have great dimensions and complex shapes. The resulting polyamide has, in comparison with the hydrolytic polyamide usually produced and used in granules, a higher molecular weight (and a higher crystallinity) and exhibits a higher modulus, a higher chemical stability to the solvents, a lower permeability to the gases and a lower moisture absorption, what results in a higher dimensional stability. The anionic polymerization process is well suited to the modification in situ of the polyamide with proper elastomerizing agents, during the formation of the polymer, increasing the impact strength thereof. The polymer so obtained shall possess, however, not only a high impact strength (resilience), but also a good homogeneity in every point of the manufactured article, no matter how great are the dimensions, as well as smooth, bright and aesthetically valid surfaces.